FacilitySource Acquisition - Certain Adjustments to Unaudited Pro Forma Results (Detail) - FacilitySource [Member] - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Depreciation expense	$ 1,090	$ 2,377	$ 3,269
Amortization expense	1,441	2,976	4,324
Interest expense	$ 1,525	$ 2,769	$ 4,574

The amount of incremental amortization expense that would have been recognized for the period if the business combination or combinations had been completed at the beginning of a period.

No definition available.

Name:	cbg_BusinessAcquisitionsIncrementalAmortizationExpense
Namespace Prefix:	cbg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration